none 10.1177/87552930221130762 Wiley Wiley (John Wiley & Sons) 311 142135012 5867 2026012504112200378 10.1002 2026-01-25T14:48:54Z 2022-11-07T09:06:43Z 17 Earthquake Spectra Earthquake Spectra 8755-2930 1944-8201 02 2023 39 1 Andrew Stolte The University of Auckland Auckland New Zealand https://orcid.org/0000-0001-5180-2247 Liam Wotherspoon The University of Auckland Auckland New Zealand Brady Cox Utah State University Logan UT USA Clinton Wood University of Arkansas Fayetteville AR USA Seokho Jeong Changwon National University Changwon South Korea https://orcid.org/0000-0001-9266-1452 James Munro The University of Auckland Auckland New Zealand An experimental study of the fundamental period of vibration of geologic strata above multiple impedance contrasts is presented for the Canterbury Plains, a deep, complex sedimentary basin on the South Island of New Zealand. Microtremor Horizontal‐to‐Vertical Spectral Ratio (mHVSR) analysis was used at 152 locations to investigate the fundamental period of the strata above three significant impedance contrasts in the region: the Riccarton Gravel, the Banks Peninsula volcanic (BPV) rock, and the deep graywacke basement bedrock. Up to three mHVSR peaks were observed in many of the mHVSR curves, with the long‐period (2–7 s) peaks controlled by the BPV and/or the underlying bedrock. As the thickness of the BPV increases, the response of the geologic strata above the basement bedrock was masked by the shallower BPV, effectively changing the definition of what constitutes the reference condition (i.e. engineering bedrock) for site response analyses. The mHVSR peaks matched the long‐period amplification peaks observed in the response spectra calculated from ground motions recorded during the Canterbury Earthquake Sequence events, underscoring the importance of the identification and consideration of deep impedance contrasts in site classification. 02 2023 02 2023 288 309 10.1177/87552930221130762 National Science Foundation https://doi.org/10.13039/100000001 872 CMMI‐1261775 CMMI‐1303595 Earthquake Commission https://doi.org/10.13039/100012181 http://onlinelibrary.wiley.com/termsAndConditions#am http://onlinelibrary.wiley.com/termsAndConditions#vor http://doi.wiley.com/10.1002/tdm_license_1.1 2 10.1177/sage-journals-update-policy journals.sagepub.com true National Science Foundation https://doi.org/10.13039/100000001 872 CMMI-1261775 National Science Foundation https://doi.org/10.13039/100000001 CMMI-1303595 QuakeCoRE https://doi.org/10.13039/100017557 Earthquake Commission https://doi.org/10.13039/100012181 http://www.sagepub.com/licence-information-for-chorus 10.1177/87552930221130762 https://onlinelibrary.wiley.com/doi/10.1177/87552930221130762 http://sage.cnpereading.com/paragraph/article/?doi=10.1177/87552930221130762 https://onlinelibrary.wiley.com/doi/pdf/10.1177/87552930221130762 https://journals.sagepub.com/doi/pdf/10.1177/87552930221130762 https://onlinelibrary.wiley.com/doi/pdf/10.1177/87552930221130762 https://onlinelibrary.wiley.com/doi/full-xml/10.1177/87552930221130762 https://pubs.geoscienceworld.org/earthquake-spectra/article/39/1/288/620220/the-influence-of-multiple-impedance-contrasts-on 10.1111/j.1365-246X.2008.04020.x 10.1016/j.earscirev.2006.07.004 10.1016/j.soildyn.2012.06.004 Geology of the Christchurch Urban Area Brown LJ 1992 NEHRP Recommended Provisions for Seismic Regulations for New Buildings and Other Structures, Volume I: Part 1 Provisions (FEMA P‐2082‐1) Building Seismic Safety Council (BSSC) 2020 10.1785/0120060095 10.1093/gji/ggaa342 10.1061/(ASCE)GT.1943-5606.0002551 10.1093/gji/ggaa119 10.5459/bnzsee.44.4.205-226 10.1193/122717EQS267M Eurocode 8: Design of Structures for Earthquake Resistance (EN 1998‐1:2004. 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